1. Field of the Invention
The present invention relates generally to an alignment system providing for the precise adjustment of the alignment of a matched filter array in an optical correlator employing the matched filter array as its memory. More particularly, the subject invention pertains to an alignment system for aligning a multiple matched filter array relative to a multiple holographic lens in an optical correlator wherein special alignment targets are added to the multiple matched filter array to provide for proper alignment thereof relative to the multiple holographic lens.
2. Discussion of the Prior Art
An optical correlation system is disclosed in U.S. Patent Application Ser. No. 814,209, filed Dec. 27, 1985, relative to which the alignment system of the present invention was developed. The optical correlation system disclosed therein optically compares an input image with optical information stored in multiple matched filters to provide identification and aspect information about the input image. In one disclosed embodiment, the input image is directed onto a spatial light modulator to spatially modulate a coherent beam of radiation. The spatially modulated radiation beam is directed onto a multiple holographic lens which performs a multiple number of Fourier transformations thereon to obtain an array of a multiple set of Fourier transforms of the spatially modulated radiation beam. A corresponding array of matched filters has the array of Fourier transforms incident thereon, with each matched filter comprising a Fourier transform hologram of an aspect view of an object of interest. Each matched filter passes an optical correlation signal in dependence upon the degree of correlation of the Fourier transform of the spatially modulated radiation beam with the Fourier transform hologram recorded thereon. An inverse Fourier transform lens receives the optical correlation outputs of the array of matched filters, and performs an inverse Fourier transformation on the optical correlation outputs. A detector then detects the inverse Fourier transforms of the optical correlation outputs, and the output thereof is utilized to determine identification and aspect information about the input image.
One problem with this type of optical correlator is that of obtaining a proper and precise alignment (x, y and .theta. rotational) of each individual matched filter with the particular Fourier transform incident thereon generated by the multiple holographic lens. A typical matched filter optical correlator is normally initially set or adjusted such that the axial distance along the z axis (optical axis), the .gamma. (pitch) adjustment, and the .beta. (yaw) adjustment remain properly set and aligned. The initial adjustments of z,.gamma. and .beta. are normally properly retained by the optical correlator, and do not require re-adjustment each time a new matched filter is placed therein. When a new matched filter is placed in a typical optical correlator, adjustments and alignments are normally required only along the x axis, the y axis, and the .theta. (roll) axis.
Burch, U.S. Pat. No. 3,539,260 discloses one arrangement for the automatic alignment of coherent optical spatial frequency filters, and uses four positional reference patterns superimposed along the orthogonal axes of a target matched filter plate, and the reference patterns are alternatively processed with the target pattern in a time division multiplex arrangement. This patent permits alignment for a limited application of one matched filter, and expresses no awareness of orientation or critical alignment for arrays of matched filters, nor does the patent disclose or use complex multiple holographic lens elements. For a system such as that disclosed in U.S. Ser. No. 814,209 which employs an array of multiple matched filters and a multiple holographic lens, this prior art approach cannot be used because all outputs are derived simultaneously and matched filter movement is required.